1. Field of the Invention
The present invention relates to a bias circuit and an amplifier capable of controlling a bias voltage according to the amplitude of a high-frequency signal input thereto.
2. Description of the Related Art
In general, a digital modulation/demodulation scheme has been selected as a wireless communications scheme, and appropriate schemes have been employed in consideration of frequency utilization efficiency. For instance, a quadrature phase shift keying (QPSK) scheme is employed for mobile telephones using a code division multiple access (CDMA) scheme, while an orthogonal frequency division multiplexing (QFDM) scheme is employed in a wireless LAN conforming to the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard.
Wireless communications systems in which such wireless communications schemes are employed include power amplifiers for amplifying high-frequency signals.
Here, in a system requiring linear amplification, a power amplifier having linear characteristics for amplifying a transmission signal without distortion is needed. Here, linear characteristics means that the power of an output signal is amplified at a constant rate while the phase thereof is not changed, even in the case that the power of an input signal varies.
In addition, a technology which compensates for temperature so as to prevent a power amplification rate from being significantly changed, even if an ambient temperature changes, is required.
In an existing power amplifier, a bias voltage generated by a bias circuit is supplied to a power amplifying element. The bias circuit may include a temperature compensation function.
In the existing power amplifier, a bias voltage and a bias current are provided to the base of an amplifying transistor included in an amplifying circuit so as to amplify a high-frequency signal input thereto.
Although the existing power amplifier has a temperature compensation function using a transistor included in the bias circuit, the temperature compensation function thereof may also be limited, such that the bias current may increase as temperature increases. In this case, the increased bias current may in turn cause the temperature at the junction of the transistor included in the bias circuit to be increased, such that the so-called “thermal runaway” phenomenon may occur.
In order to overcome this shortcoming, a current limiting resistor for limiting the bias current may be provided between the current supplying terminal of the bias circuit and the base of the amplifying transistor.
However, in the existing power amplifier, when a high-frequency signal having a higher amplitude than a normal amplitude is input thereto, the bias current increases according to the characteristics of the amplifying transistor. As the bias current increases, the voltage drop across the current limiting resistor is increased, such that the voltage supplied to the base of the amplifying transistor may be reduced.
As described above, when a high-frequency signal having a relatively large amplitude is input, the bias voltage supplied to the base of the amplifying transistor may be reduced, such that a linear region in which the amplifying transistor may perform amplification without distortion is narrowed. As a result, the high-frequency signal having a large amplitude may be distorted.
Patent Document 1 referenced below relates to a power amplifier but does not teach controlling a bias voltage according to the amplitude of a high-frequency signal.